TWINS
 

For a year or so I've been thinking about a twin RC model. I've only built one twin, many years ago. It was a CL P-38 that I designed, powered by two cox .010's. It flew surprisingly well. I have no other experience with twins.
I've ridden in a number of PBY's (its a beautiful plane), and that seems to be a good, logical first twin.
It's a large, slow, high wing, with engines very close together, it wasn't too far from a powered glider. I think it cruised at about 100kt. and landed at 80kt.
I have four engines that I like, two are PAW .19 "diesels", and two "pre China" ST-.34's. and I'm wondering about the size of the plane if I try to build light, and fly in a scale manner.
Pulling some numbers from the ether via Mr. Lennon I'm guessing at the following;

Power..........................2 @ .19 diesel @ 6.5 oz. ea.
Props ..........................10X4 (recommended)
Weight ....................... 6.25 lb. = 100 oz.
Wing loading ................0.14 oz./sq. in. =20 oz./sq. ft
Wing area.....................720 sq. in. (5 sq. ft.)
Lift. coeff. ...................1.00

Construction:
Built up fuselage and tail; wing built up partially sheeted or sheeted foam,

Haven't looked at airfoils yet but I should be able to find one that will give a lift
coefficient of 1 at 20-25 mph.

I'll appreciate any comments and suggestions.

Thanks

RE: TWINS
 

This is my bashed twin. It is built from a Sonic ARF. Flies nice with two 15 LA's. Maybe a tad small for the .19's. It is very aerobatic, yet well behaved at lower speeds.

http://www.rcuniverse.com/forum/m_14...tm.htm#1437936

RE: TWINS
 

The numbers look pretty good to me. If you can assure yourself that you can build light then perhaps bump the size up to get 800 sq inches. 800 at 6 to 6.5 lbs would make the model fly visually just like the full sized ones I've seen. To do this you may need to loose the balsa over foam method and adopt some more open structures similar to RC assist old timers and the Sig Kadet Senior. The fuselage will be the hard part what with the various compound curves of the PBY.

Perhaps a built up frame and sheet style. One method I've always wanted to try is carving a blue or pink styrofoam plug and then molding balsa over it using strong ammonia to plasticise the balsa. Various panels would then be molded over the foam plug and after drying would be transfered to the structure made up of formers and a few key stringers. This method would avoid much of the weight of glue and excess balsa of a typical planked structure.

Another option would be to carve the plug as above and lay up a LIGHT 4 oz epoxy-glass skin over it. Then carve out all but a 1/4 inch layer of foam from the inside and then lay up another LIGHT 4 oz inside layer. This will produce a light "honeycomb" fabrication that my just rival a lightly built up balsa version but with potentially a lot less work. I would make up the form using formers and keels that are sized to the 1/4 final thickness. Blocks of foam would then be tack glued between the formers and the whole works carved to look like a PBY. The formers and keels would provide the guide for carving to shape. Then, after the outside glass work is done, the form could be split with a fine saw and the insides hollowed out down to the formers and keels and glassed in as well. Hard points in the skin could be added between the two skins and finally some of the key insides cut from the shell formers would provide the inside formers for structural needs and some inside support.

But all this is getting away from the aerodynamics aspect....

For your airfoil I would suggest cheating and using one of the modern model sailplane options. I know from personal experience with an electric glider that the Eppler 201 is a good airfoil that works very well with a higher loading. Although I have not tried a model with it yet I think the Selig 4233 or S7062 would be a great choice also. But in any event you definetly want something in the 12% thickness range and with 3 to 4% camber. Or if you want to be able to really slow it down yet still retain a decent cruise speed something with "pre-deployed flaps" like the Goetingen 501 or 549 or similar higher cambered options would work nicely. I've got an old timer RC assist model that uses the 501 and I'm quite surprised at how nice a speed range it shows. It'll hang nicely but if I push some down trim into it it looses about 15 feet as it speeds up then levels out and speeds along with a nice flat glide until I ease off the trim at which time it gets most of the original 15 feet back again. At just over 1000 sq inches and at 5.5 lbs it flies pretty much as I'd like to see a PBY fly in a scale like way. The higher camber options will also easily do more than a Cl = 1. Typically for similar shapes I've seen then pull up to 1.4 or 1.5.

RE: TWINS
 

Thanks for your response gentlemen.
AQ500:
Beautiful! That looks like a pretty hot twin, I'm wondering what the weight and wing area came out to be.

Mr. Mathews, I have plans for a PBY by one of your countrymen. I saw photo's in several magazines and contacted the builder.
The numbers in my post were based on the two .19 diesels, but I may make the nacelles large enough for the two ST-.34's, just in case.
I agree about the 800 sq inch wing. I'll get serious about the numbers now.
I have a Kadet Senior that I am modifying to resemble a cub, and I have always liked that kind of light construction, my original thought was a built up fuselage with a balsa sheet/plank skin. I'm pretty comfortable with that, I've never done much with glass on a model.
I do plan to use a lot of high strength and composite materials to reduce weight.
Haven't had much luck with Amonia and balsa however I've only used "grocery store" amonia. I might try to get some higher concentration from the local blueprint company.
As far as "cheating" on the airfoil is concerned. I have no problem with that. I don't even have a problem with flaps!
I'll take a look at the airfoils you mention thank you.
It's nice to have other modelers to bounce Ideas off of before you spend a lot of time and effort!